The present invention relates to a method and an apparatus for detecting failure of nuclear fuel, and particularly to a method and an apparatus for detecting failure of nuclear fuel for a shorter time.
A large number of fuel assemblies are disposed in a rector core of a boiling water type nuclear reactor. When fuel rods constituting fuel assemblies are damaged, a radioactive material flows out of the fuel rods and contaminates the cooling water passing through the reactor core. When a nuclear reactor is operated in such a state as a large number of fuel rods remain in failure, a nuclear reactor pressure vessel surrounding the reactor, structures within the pressure vessel, and apparatuses and pipings of nuclear reactor plant such as a cooling water purification apparatus, a steam turbine, etc. are contaminated by the radioactive material of a high radioactivity level, and their maintenance and inspection are difficult to conduct. Furthermore, there is a risk of increasing the radioactive material that will be discharged into the surrounding atmosphere through an off-gas system connected to a condenser. To avoid such a risk, the state of failure of fuel assemblies charged in the reactor core is periodically inspected, and as soon as a failure of nuclear fuel assembly is found, it is replaced with a new fuel assembly.
The conventional method for detecting a failure of fuel assembly is disclosed in U.S. Pat. No. 4,147,587. The outline of the method is given below:
After a boiling water type nuclear reactor is shut down, the top cover of a nuclear reactor pressure vessel is removed, and cooling water is filled therein up near to the topmost floor of nuclear reactor housing. A sipper cap of an apparatus for detecting failure of nuclear fuel is fixed to the top part of fuel assemblies charged in the reactor core. In parallel with the sipper capping operation, a flushing operation is carried out for a sampling line fixed to the shipper cap for sampling the cooling water in the fuel assemblies (sampling line flushing operation). Flushing-out of radioactive material, etc. deposited in the sampling line during the preceding sampling operation is carried out by sampling line flushing operation. After the end of sipper capping operation, air is supplied into the sipper cap to form an air layer, and a fuel assemblies to be inspected are isolated from other fuel assemblies (isolation operation). The flow of cooling water passing through the fuel assemblies is stopped by an isolation operation. The isolation operation is continued until a sampling operation, which will be described later, has been completed. After discharging air bubbles at a recess part at the lower end of the sipper cap is confirmed, the fuel assemblies are kept in that state for a predetermined period of time (soaking operation). The air is supplied into the sipper cap during the isolation operation. During the soaking operation, the temperature of cooling water in the fuel assemblies is increased by the heat of decay of nuclear fuel. If there is failure of fuel rod in a fuel assembly, the radioactive material is discharged from the part in failure of the fuel rod, and the radioactivity level of the cooling water in the fuel assembly is increased. During the soaking operation, a sampling pump flushing operation is completed. The sampling pump flushing operation is a flushing operation for the downstream side of sampling line including a sampling pump, which has not been subjected to the sampling line flushing operation. As soon as the soaking operation is completed, the sampling pump is driven to discharge te flushing water (cell water) retained in the sampling line therefrom at a discharge outlet (cell water removal operation). At that time, a portion of the cooling water in the fuel assemblies is led by suction into the sampling line projected into the fuel assemblies through the sipper cap at the suction inlet. When the removal of cell water is completed, the driving of the sampling pump is stopped, and a sample water receptacle is set to the discharge outlet of the sampling line (sample water receptacle-setting operation). Then, the sampling pump is driven for a predetermined period of time, and the cooling water in the fuel assemblies is sampled. A predetermined amount of the sampled cooling water is led into the sample water receptacle from the sampling line at the discharge outlet (samping operation). The radioactivity level of the cooling water in the sample water receptacle is measured, and the presence of failure of the fuel assemblies is determined by the measured radioactivity level. When the sampling operation is completed, a sipper cap is fixed to other fuel assemblies, and the aforementioned operations are successively repeated.
In the foregoing conventional method for detecting failure of nuclear fuel, operating personnel must set the sample water receptacle to the discharge outlet of the sampling line. The setting of the sample water receptacle is carried out after the discharge of the cooling water at the discharge outlet has been completely stopped to prevent a risk of radioactive exposure of the operating personnel through contact with the cooling water discharging from the sampling line at the discharge outlet.